Commercialization of the Internet
The Internet used to be an esoteric little toy for the military and scientists who wanted to share information and to prevent that information from being lost in case of nuclear war.
However, the Internet experienced explosive growth only when it became commercialized starting around 1995. Without the influx of money of private companies in the form of advertising revenue and the improvements in the infrastructure (backbone, routers, etc.), I doubt that the Internet could have expanded as fast as it has.
With commercial interests looming over the Internet, advertising was inevitable. In fact, advertising on the Internet has become somewhat like advertising on the Superbowl -- people come for the ads themselves instead of the content. With the blurring of the distinction between advertising and content, the Internet is conducive to this desire for the ads instead.
What better evidence than people willingly, on their own dime, going to visit a website promoting a movie? The website could contain movie clips, interviews with the actors, promotional stills, and a plot summary. Or, perhaps, websites for shows like Star Trek?